throbscottle
Well-Known Member
I'm in a bit of a quandary with the input circuit for my little bench dmm project.
I've designed it so far so that the most sensitive range, 2.5v (or 250mV with different scaling in the uC) connects the adc input directly to the input socket of the meter, using a reed (or other, more robust) relay.
The more I look at it, the more this seems a Bad Idea to me, for the simple reason that if a voltage higher than the relay's contact rating is applied to the input, it could arc over and blow everything up.
I did think about:
putting two relays in series, though it seems a severe kludge,
using an extra resistance in series with the adc input,
or having a high voltage relay (expensive).
I also thought about using the first tap on the voltage divider so there is a permanent divide by 10, changing the scaling in the uC and losing the μV capability, but where's the fun in that?
So the next most logical thing I could think of was to use the always divide by 10, but then have an ins. amp with a gain of 10, and put up with the extra possible source of errors.
Bearing in mind that I'm a hobbyist with no way to do anything particularly precisely, and a very limited budget, what does anyone think?
(While I'm on the subject, I'd like to give this project some kind of name to publish the design as - suggestions?)
I've designed it so far so that the most sensitive range, 2.5v (or 250mV with different scaling in the uC) connects the adc input directly to the input socket of the meter, using a reed (or other, more robust) relay.
The more I look at it, the more this seems a Bad Idea to me, for the simple reason that if a voltage higher than the relay's contact rating is applied to the input, it could arc over and blow everything up.
I did think about:
putting two relays in series, though it seems a severe kludge,
using an extra resistance in series with the adc input,
or having a high voltage relay (expensive).
I also thought about using the first tap on the voltage divider so there is a permanent divide by 10, changing the scaling in the uC and losing the μV capability, but where's the fun in that?
So the next most logical thing I could think of was to use the always divide by 10, but then have an ins. amp with a gain of 10, and put up with the extra possible source of errors.
Bearing in mind that I'm a hobbyist with no way to do anything particularly precisely, and a very limited budget, what does anyone think?
(While I'm on the subject, I'd like to give this project some kind of name to publish the design as - suggestions?)